We are investigating the role of reactive oxygen species (ROS), such as hydrogen peroxide and superoxide anion, as specific regulators of signaling in the adaptive immune system. Phagocytes produce large amounts of ROS in response to infectious or inflammatory stimuli through the prototypic NADPH oxidase (Nox) containing gp91phox (Nox2). Recent discovery of multiple homologues of gp91phox (Nox1, Nox3-5, Duox1, Duox2) has opened studies on the roles of Nox-derived ROS in non-phagocytic cells. In non-phagocytic cells, Nox family members produce lower levels of ROS that can act as signaling molecules. Our studies of the functions of Nox family members in the lymphocyte provide opportunities to establish distinct roles of deliberate ROS generation in adaptive host defense response to diverse pathogens. The roles of ROS in regulation of T cell activation and function are not clear and may involve several different Nox family oxidases in specific contexts. We demonstrated functional expression of Nox2 in T cells. T cells from Nox2-deficient mice and humans have defective ROS production, exhibit enhanced ERK activation and a relative increase in Th1 cytokine secretion. Thus, mature T cells express a functional phagocyte-type NADPH oxidase that appears to downregulate elements of TCR signaling. However, an early phase of ROS generation in T cell receptor (TCR)-stimulated cells was found to be independent of the phagocyte oxidase. We found that Duox1, a calcium-dependent Nox family member, is responsible for early TCR stimulated ROS generation. Activation of Duox1 is dependent on early tyrosine kinase signaling, PLC-gamma1 and intracellular calcium release. In 2009, we found that knockdown of Duox1 in primary human CD4+ T cells and Jurkat T cells selectively inhibits ROS generation and phosphorylation of proximal signaling molecules, suggesting a positive role in TCR signal transduction. Specifically, phosphorylation of Y319 in ZAP-70 kinase and downstream activation of ERK kinase were strongly inhibited and these effects were reversed upon re-expression of Duox1. TCR stimulation induced oxidation of protein tyrosine phosphatase SHP-2, which was inhibited in cells having stable knockdown of Duox1 expression. The data support a model in which Duox1-mediated SHP-2 oxidation affects proximal TCR signaling through effects on phosphorylation and activation of ZAP-70. The results suggest that ROS produced by Duox1 are key mediators of TCR signal transduction that are dependent on proximal TCR signals and act in a positive feedback loop to promote proximal and downstream mediators of TCR signal transduction.